Also, comparisons between X-33 and X-34 are totally inappropriate: X-33 was meant to demonstrate the feasibility of SSTO (or put a final nail on its coffin - that would have been a very useful result). X-34 was meant to measure the cost of reuseability of a reuseable first stage (in terms of parts to be replaced, labor required for turnaround, degree of inspection and retesting required, etc.) Vital data to validate the non-mass-fraction costs of reusability for which there is still only ONE data point.

Huh? The airframe is not designed for a hybrid. How many times does it have to be said that these systems are not Lego pieces.

Well, a bit more justifiable here in that the engine that X-34 was designed around does not (and will not) exist. So, any engine will require some level of redesign.

A hybrid would be cheaper to operate, but require more modifications to the airframe. And if they are really planning to use it as a testbed for a reusable EELV-replacement first stage, I don't think a hybrid is likely, as they'd likely also be using it as an engine testbed. If the goal is just approach and landing systems tests, a hybrid is more likely...

Huh? The airframe is not designed for a hybrid. How many times does it have to be said that these systems are not Lego pieces.

Well, a bit more justifiable here in that the engine that X-34 was designed around does not (and will not) exist. So, any engine will require some level of redesign.

A hybrid would be cheaper to operate, but require more modifications to the airframe. And if they are really planning to use it as a testbed for a reusable EELV-replacement first stage, I don't think a hybrid is likely, as they'd likely also be using it as an engine testbed. If the goal is just approach and landing systems tests, a hybrid is more likely...

If the goal is just approach and landing systems tests, then just drop it from an airplane.

Hybrids aren't really that great.

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Well, a bit more justifiable here in that the engine that X-34 was designed around does not (and will not) exist. So, any engine will require some level of redesign.

A hybrid would be cheaper to operate, but require more modifications to the airframe. And if they are really planning to use it as a testbed for a reusable EELV-replacement first stage, I don't think a hybrid is likely, as they'd likely also be using it as an engine testbed. If the goal is just approach and landing systems tests, a hybrid is more likely...

A solid is more likely (still not what's going to happen, but more likely). If your objective is to test the feasibility of a rapid-turnaround, reusable first stage, then don't saddle it with an unrelated immature technology. That is, you don't test your new brake design on a vehicle with articulating RadiKS Smartwheels.

According to John Kelly, NASA Dryden's Flight Opportunities Program office manager, the two vehicles are under consideration as potential flight platforms for reusable space launch vehicle technology demonstrations, such as return-to-launch-site recovery and rapid-turnaround for lower-cost operations.

For the recovery part, you might not need an engine, depending on how much of the envelop you want to simulate. The whole point, though, of a flyback stage is that it is flightworthy from 2-3 km/s burnout to landing, so I'd think you'd want some sort of engine. The rapid-turnaround part also rather confines the fuel to RP-1/LOX to be applicable down the line.

I'm not sure a flight demonstrator budget would be sufficient to cover the costs of adapting NK-39 for US use; are there any small RP-1/LOX, <100 kN thrust engines currently available? Two or three of those would be flexible drop-in replacement for FASTRAC...

According to John Kelly, NASA Dryden's Flight Opportunities Program office manager, the two vehicles are under consideration as potential flight platforms for reusable space launch vehicle technology demonstrations, such as return-to-launch-site recovery and rapid-turnaround for lower-cost operations.

For the recovery part, you might not need an engine, depending on how much of the envelop you want to simulate. The whole point, though, of a flyback stage is that it is flightworthy from 2-3 km/s burnout to landing, so I'd think you'd want some sort of engine. The rapid-turnaround part also rather confines the fuel to RP-1/LOX to be applicable down the line.

I'm not sure a flight demonstrator budget would be sufficient to cover the costs of adapting NK-39 for US use; are there any small RP-1/LOX, <100 kN thrust engines currently available? Two or three of those would be flexible drop-in replacement for FASTRAC...

Merlin is available, but it is a bit over powered. Still it is the cheapest/smallest US RP-1 engine available without a new engine development program, which I would assume the AF is trying to avoid for a technology demonstrator.

Edit: I suppose the NK-33 could be used as well, but since Merlin is in production and already flight tested on multiple flights.......

The big thing is that the AF will probably use a "off the self" engine, while things like TPS and the rocket back maneuver will be the big tests if the X-34 can test the later.

Merlin is available, but it is a bit over powered. Still it is the cheapest/smallest US RP-1 engine available without a new engine development program, which I would assume the AF is trying to avoid for a technology demonstrator.

Edit: I suppose the NK-33 could be used as well, but since Merlin is in production and already flight tested on multiple flights.......

The big thing is that the AF will probably use a "off the self" engine, while things like TPS and the rocket back maneuver will be the big tests if the X-34 can test the later.

The Merlin probably would be the best engine to use as it would require the least amount of work and it has modern control systems.

The RS-27 is way too big and the NK-39 probably would require development of modernized control systems for the engine.I could not see them dealing with 1970 era controls that lack comprehensive health monitoring and use outdated data buss standards or worse yet analog controls.

No matter what they use there will be modifications to the thrust structure of the airframe and CG change issues.

The RS-27 is way too big and the NK-39 probably would require development of modernized control systems for the engine.I could not see them dealing with 1970 era controls that lack comprehensive health monitoring and use outdated data buss standards or worse yet analog controls.

Don't forget that Orbital was baselining NK-39 for much of the development phase of the project. Also, Orbital now has quite a bit of experience integrating NK engines into their systems.

Has Merlin ever been test fired on it's side? Does SpaceX has the extra engineering resources for this one off to make it work in this unique mode?

I vaguely recall from one of the RL-10 histories, that they did a bunch of the initial testing of it on it's side, and when they tried to fire it vertically for the first time on a test stand, they ran into trouble and had to rework it. (I am not advocating a LH/LOX switch, the vehicle was designed for Kero/LOX).

Does anybody know of any articles which discuss what happened during the X-34's cancellation? I'd like to add info about it to the X-34 wikipedia article, which currently only has some unreferenced claims.

The closest I've been able to find is this snippet from a 2003 Clark Lindsay interview of Gary Hudson:

HS: There doesn't seem to be much of a tradition of investigative journalism within the aerospace press. When you talk about your case and the X-37 situation, it reminds me of the X-34 cancellation, which I saw as an outrage if not a scandal. As I understand it, in the aftermath of the two Mars mission failures, all major projects got hit with system reviews to avoid any more embarassing public disasters. For the X-34, they added so many additional safety requirements that it crossed their cost limit and so was canceled. This happened even though, in traditional X project spirit, Orbital had built 3 vehicles just so it would have backups if one was lost. In addition, NASA had required that they use the Fastrac engine, which as far as I know, was a project that simply faded away without ever flying an engine or making any public accounting whatsoever.

You surely know more about what really happened in these cases than I do. However, the fact that I have to guess as to what happened [in these and your cases] rather than go to a NASA web page and read a clear and full summary of such projects [and how decisions regarding them were made] absolutely amazes me. And I'm even more amazed that the aerospace press lets them get away with it.

Hudson: You have the story exactly right, insofar as I am aware. While I am not a fan of Orbital, I have often taken their side in the X-34 matter. NASA MSFC needs a through housecleaning and it requires a persistent and competent press to get Headquarters or the Congress to do their job.

Let me add my two cents' worth of history concerning the birth and death of X-34:

Sometime around 1993-4, there was a big NASA conference concerning what should be done to capitalize on the success of DC-X and a general feeling that the shuttle was too expensive as a means of attaining orbit, and that a cheaper alternative should be developed. As a result of this conference, the idea of a SSTO X-vehicle was mooted, which became X-33. Somehow, and to the great amazement of a lot of people, X-34 also emerged at the same time. Many did not understand why X-34 was to be, the official story was that X-34 would be a pathfinder for X-33, a story told with a straight face by NASA officials who at the same time stated that X-33 would likely fly before, or in parallel with X-34, leaving open the question of how X-34 could be a pathfinder for a vehicle that would before X-34.

The scuttlebutt at the time was that this was the way to get Rockwell and Orbital to line up with the general direction that NASA was taking, although I have no idea if this is true.

At this point, things got a little murkier - it was at this moment in history that the satellite telephone constellations were emerging, and it appeared that a cheap small satellite launcher might be a significant moneymaker. And, it turned out that the initial Orbital/Rockwell X-34 concept could be an effective Iridium/Globalstar delivery vehicle. Given that X-vehicles weren't supposed to be operational commercial prototypes, there was some opposition to the X-34 in that particular incarnation. At that point, it appeared that the opposition was enough to kill X-34, and it died.

But wait, we're just getting started.

Somehow, just a short time down the road, Orbital pitched a new version of X-34, as a pure experimental vehicle, and AFAIR, using Marshall's new Fastrac engine for propulsion (Rockwell was now out of the mix). This new concept, using single string avionics for cost savings, was to be an operational demonstrator, but not suitable for commercial use. As such, it was given ATP, and in fact, Orbital began design and fabrication of the new vehicle.

Well, MSFC had some problems with Fastrac, so Orbital began looking for a replacement, and came up with using the Russian NK-39 engine. At the same time, there was a proposal from Orbital to fly at least part of the program from Florida, as opposed to Edwards. Neither of these two alternates went very far.

Meanwhile, an engineless X-34 was officially rolled out at Edwards, and a program of taxi and glide tests was expected, while the engine problem was being worked. It was at this moment of time that the telephone satellite ventures all pretty much died, so there was no prospect of an X-34 follow-on having any sort of a market. Given that X-34 program funding was running out, there was no engine, the discovery by NASA that X-34 was a risky X-vehicle was the coup de grace - in the aftermath of the two Mars probe failures, NASA decided to abandon "better faster cheaper" and instead decided to focus on safety, thereby killing X-34 again, at a point in time where X-33 was already dead, and no one really cared anymore about X-34.

There probably would have been and would be a benefit from an X-vehicle designed to test robust operations. I don't know if X-34 was or could be that vehicle, though.

I was thinking of that, too. I somewhat doubt they use that stand anymore. It has a pretty rough-built look to it.

Actually, I'd be hesitant to draw too many parallels from that engine to the current Merlin. That's the ablatively-cooled nozzle, and I don't think it even has a turbopump. It's normally pretty distinctive and both propellant lines go in and out...in the picture both propellant lines seem to run separate and I don't think the pump and associated plumbing could hide behind the chamber. I think it's a pressure fed test of the injector, chamber, throat, and nozzle.

But I miss the point of the question - why would there be a need for it to fire horizontally? I'm not real well versed on the X-34, so I apologize if I'm missing something obvious.